The present invention relates to a cover tape for packaging, and more particularly to such a cover tape adapted for use with a carrier tape to encase electronic components or other parts in the application of surface mounted technology.
Previously, container means molded from plastics were coupled with adhesive coated films for encasing electronic components so as to protect electronic components during delivery. Recently, in order to promote the efficiency of electronic component surface mounted technology; tape-like electronic component package means having pockets for accommodating electronic components are intensively used for electronic component packaging. Exemplars of these tape-like electronic component package means can be found in the U.S. Pat. Nos. 4,724,958 and 4,963,405. According to the U.S. Pat. No. 4,724,958, a tape-like electronic component package is provided, which comprises an elongated receiving tape having a plurality of cavities adapted to hold a respective electronic component and a cover tape means enclosing the cavities. According to the U.S. Pat. No. 4,963,405, a tape for encasing electronic parts is provided, which comprises a carrier tape having a plurality of recesses for encasing electronic parts, a plurality of guide perforations, and a cover tape pasted onto the carrier tape with an adhesive so as to seal only the recesses while leaving the guide perforations open. Currently, chip type electronic parts used for surface mounting are commonly supplied by being contained in a tape-like package consisting of a carrier tape having a plurality of recesses for encasing chip type electronic parts and a cover tape sealed to the carrier tape. The electronic parts contained in the tape-like package are automatically taken out after peeling the cover tape off the tape-like package, and are mounted on the surface of an electronic circuit substrate. In actual practice, the cover tape is coated or adhered with an adhesive layer adapted for fastening to the border area of the carrier tape. The material of the adhesive layer can be selected from the group consisting of the pressure sensitive adhesives and the heat sealable thermoplastics. The adhesive layer must provide proper adhesion strength so that the cover tape can be firmly bonded to the carrier tape, and then can be also smoothly peeled off from the carrier tape for permitting the encased electronic parts to be taken out from the carrier tape. However, it is not easy to have an adhesion layer between the cover tape and the carrier tape providing adequate adhesion strength and a low peeling-off strength simultaneously, because both factors are usually work against each other. Excessively high or low adhesion strength or uneven adhesion strength may cause certain problems. For example, when the adhesion strength is excessively low, the sealed cover tape may be separated from the carrier tape due to the unexpected minor bumping at the time of the package transfer, and the encased electronic components may drop off. When the adhesion strength is excessively high, the much higher force must be applied to the cover tape to peel it off from the carrier tape, thereby causing a step motion and a subsequently vibration problem. When the vibration occurs, encased electronic components may be forced to jump out of the carrier tape. More particularly, electronic components to be surface-mounted such as liquid crystal display chips, diodes, passive components including resistors, conductors and capacitors, and active components including integrated circuit, etc., have come to be made in chips of higher capability and smaller size. Minor vibration may cause the small electronic components to fall off from the carrier tape. If adhesion strength is not uniformly distributed, the both aforesaid problems may occur when peeling off the cover tape from the carrier tape. In order to eliminate the aforesaid problems, there are limitations on material selection of the cover tape and the fabrication of the cover tape. Sealing temperature, sealing pressure and sealing speed must be taken into account if the cover tape is made of the heat sealable adhesives or thermoplastics when bonding the cover tape to the carrier tape. The sealing of the cover tape to the carrier tape also has a great concern with the setting time of the thermoplastics because of their rheological properties. Generally, a faster sealing process requires a relatively higher sealing temperature, and the material must have a relatively higher coefficient of heat conductivity or shorter setting time. However, polymer is basically a good heat insulating matter. This poor heat conductive property is contrary to the short setting time requirement. If there is to add an additive having a relatively higher heat conductivity, other negative effects such as an opaque feature due to the light scattering from the different phase domains may occur. All these factors affect the development of candidate materials. It is indeed a challenge to develop an adhesive layer material for the cover tape that provides the suitable adhesion strength, achieves satisfactory packaging effect, and is also suitable for use in different processing conditions. Further, when peeling off the cover tape from the carrier tape, the unexpected abnormal peeling-off effects such as creaking in the unexpected direction must be avoided. These abnormal defects may cause the cover tape to tear off or break at the improper locations, or to have a part remained and covered on the recesses of the carrier tape. All these conditions affect the manufacturing process and productivity.
FIGS. 1A and 1B show a cover tape for packaging chip type electronic parts according to the U.S. Pat. No. 5,208,103. The cover tape 21 comprises a biaxially oriented film 22, an intermediate layer 23, a first adhesive layer 24 sealed between the biaxially oriented film 22 and the intermediate layer 23, and a second adhesive layer 25 laminated between the intermediate layer 23 and the carrier tape 6. The cover tape 21 is specially designed so that the intermediate layer 23 has weaker cohesive strength. When the cover tape 21 is heat-sealed to the carrier tape 6, a heat-sealed portion 26 is formed in the cover tape 21, and the adhesion force of the heat-sealed portion 26 is greater than the cohesive strength of the intermediate layer 23 when the cover tape 21 is peeled off from the carrier tape 6. Therefore, the peeled-off interface is within the intermediate layer 23, and the tearing force required is about 10-120 grams per bonded millimeter. However, when the cover tape 21 is peeled off after it has been bonded to the carrier tape 6, the peeling direction of the cover tape 21 has a concern with the tearing force, the peeling angle and direction, the peeling speed, and the heat-sealed portion 26. Therefore, the peeling direction may be affected by the aforesaid factors to cause an improper peeling, for example, the cover tape 21 may be peeled off from the carrier tape 6 at a angle to the machine direction or at an angle to the bonding line, causing the cover tape 21 to break improperly or the manufacturing process to stop.
FIGS. 2A and 2B show a cover tape for packaging electronic chip according to the U.S. Pat. No. 5,346,765. The cover tape 31 comprises a biaxially oriented film 32, an intermediate layer 33, a first adhesive layer 34 sealed between the biaxially oriented film 32 and the intermediate layer 33, and a second adhesive layer 35 sealed between the intermediate layer 34 and the carrier tape 6. A heat-sealed portion 36 is formed in the cover tape 31 when the cover tape 31 is heat-sealed to the carrier tape 6, and the adhesion force of the heat-sealed portion 36 is greater than the adhesion force between the intermediate layer 33 and the second adhesive layer 35 when the cover tape 31 is peeled off from the carrier tape 6. Therefore, the peeled-off interface exists between the intermediate layer 33 and the second adhesive layer 35, and the tearing force required is about 10-120 grams per bonded millimeter, i.e., the adhesion strength of the cover tape 31 to the carrier tape 6 is about 10-120 grams per bonded millimeter. This weak adhesion strength may cause the cover tape 31 to be separated from the carrier tape 6 due to a minor impact during a transfer of the package. When the aforesaid adhesive layers are designed for heat-sealed packaging, the hot melt elastomers or thermoplastics and their mixed additives which are used for the adhesive layers usually have the potentially reactive functional group or the thermally unstable functional group. Generally, the hot melt block copolymers such as the styrene-isoprene-styrene (SIS), the styrene-butadiene-styrene (SBS) and the butyl rubber (BR), and the blended resins usually have an unsaturated bonding structure. The heat-sealable thermoplastics used to have the additive with the structure featured either a low glass transition temperature or thermally unstable functional group, for example, the thermal unstable functional group of the ester linkage, may be caused to decompose easily by heat. Materials having the unstable functional group, which is unstable due to moisture or heat, tend to be affected by humidity or temperature, or both of them. For example, because of the nature of the hydrogen bonding, the material having the carboxyl function group tends to deteriorate upon the effect of humidity and high temperature. Ethylene-vinyl acetate (EVA) either blending with other material or copolymerizing with the other material having the ester linkage is a functional group sensitive to high temperature and humidity. The aforesaid materials tend to be affected by weather, storage temperature and humidity. The material properties may change over time, more particularly the speed of material property deterioration is accelerated when under high temperature and high humidity conditions simultaneously, thereby causing the materials to deviate from their application condition and the shelf life of the materials to be shortened. It is common that the physical properties of the adhesive materials start to deviate from their application range at a certain length of time after the storage of the materials. If the adhesion strength is changed to an excessively low level, the cover tape may be separated from the bonded carrier tape causing the encapsulated parts to drop off from the recesses. On the other hand, if the adhesion strength is changed to an excessively high level, the applied peeling force may not be uniformly distributed over the cover layer when the cover tape is peeled off from the carrier layer. Thereby, it causes the carrier tape to vibrate and a consequent jump-off problem of the sealed parts.
FIG. 3 shows another structure of cover tape according to the prior art. The cover tape 41 comprises a biaxially extended film 42, an intermediate layer 43, and a back layer 47. The biaxially extended film 42 and the intermediate layer 43 are bonded together by means of a first adhesive layer 44. The intermediate layer 43 is bonded to the carrier tape 6 by a second adhesive layer 45. The back layer 47 is bonded to the intermediate layer 43 by the second adhesive layer 45. The biaxially extended film 42 has longitudinal score lines 46. The cohesive strength along the longitudinal score lines 46 is the weakest. The longitudinal score lines 46 can be scored prior to be sealed with the carrier tape 6 or during the sealing process. When the cover tape 41 is peeled off after it has been bonded to the carrier tape 6 by a heat press, it is torn along the longitudinal score lines 46. This structure may provide a better adhesion strength to the carrier tape 6. Unfortunately, a precision scoring process must be employed to cut the biaxially extended film 42. The application of the precision scoring process requires a cutting tool, a pressure and temperature control system. Because of weak cohesive strength along the scored lines 46 and the effect of thermal shrinkage of the film, the scored depth of the longitudinal score lines 46 and the ratio of depth of the scored line 46 and thickness of the film affect actual application of the cover tape 41. This longitudinally grooved, biaxially extended film 42 is fragile to impact force and stress build-up due to the effect of thermal shrinkage.
Practically encased objects of either electronic active components or passive components use to be brought together on the printed circuit board. Active components are usually sensitive to the static charge. Static charge impact can easily damage the active components during packaging or transferring. However, static charge impact to impose on the active components may occur via the contact with a charge carried passive component or via the charge induction from the other charge carried components indirectly. Therefore, an antistatic or charge dissipation treatment is needed.
The U.S. Pat. Nos. 5,441,809 and 5,599,621 describe a cover tape structure for the surface mounting device packaging that has a peeling force of between 30 and 80 grams per millimeter after heat-sealed to a carrier tape to form a two-piece package for the electronic parts. The cover tape comprises a polymeric film, one side of which is coated with a layer of metal, and a heat sealable adhesive is laminated to the metal-coated side of the film, the adhesive layer comprising thermoplastic elastomer, metal powder or the metal coated particle. Because of the certain characteristics of the material compatibility, phase separation may occur if this blend material is used under the high level of humidity and temperature over the elapsed time. Therefore, the physical property of this adhesive layer, such as the peeling strength, can deviate from within 30-80 grams per millimeter to the improper application range. If the peeling strength is too low, the cover tape can be loosen from the carrier tape during the packaging and processing and the packaged parts can be lost. If the peeling strength is too high, then the carrier tape can have the jumping-off problem due to a step motion detaping operation and the packaged part lost or position misplaced. Further, due to the chemical composition of the adhesive layer, the cover tape may be suitable for sealing a carrier tape made a particular material, for example, the polystyrene.